The present invention relates to a pressure control device, and more particularly to a pressure control device for hydraulic brake systems.
Conventional hydraulic automotive brake systems utilize a source of hydraulic pressure to actuate a piston to bias a brake shoe having a friction material surface into contact with a brake drum in a drum brake system or to bias a piston to move a brake pad containing friction material into contact with a vertical face of a rotor in a disk brake system. Because of the out-of-roundness inherent in brake drums and of the lateral run-out inherent in rotors, the friction element alternately engages low and high spots on the brake drum or rotor. Because of the relative incompressibility of the brake fluid the pressure in the brake system experiences a sharp increase when the friction element engages a high spot on the drum or rotor and experiences a relative pressure decrease when the friction element rubs a low spot on a drum or rotor. These pressure fluctuations which occur in the brake fluid cause pressure waves, surges, spikes and harmonics to propagate throughout the hydraulic system.
Typically, a pressure wave would move from a wheel cylinder or disk brake piston to the brake master cylinder and thereafter be reflected back from the master cylinder to the wheel cylinder or brake caliper piston. Very high momentary braking pressures occur within the hydraulic system when the reflected pressure waves, surges, spikes and harmonics moving toward the brake cylinder or piston add to clamping force already exerted thereon.
In common hydraulic automotive brake systems one or more wheels of the vehicle may lock or skid during severe braking applications while the other wheels are rotating, which may cause the vehicle operator to experience a loss of control. It may be demonstrated that wheel lockup occurs because a friction element becomes "stuck" on a so-called high spot on a disk brake drum or rotor. This wheel lockup occurs because the high spot initiates a high pressure wave into the hydraulic system which moves from a wheel cylinder or brake piston towards the master cylinder and reflects back through the brake line and adds to the clamping force already exerted on the shoe or caliper. Consequently, it has been found that the addition of a small accumulator to the hydraulic system will absorb pressure surges to maintain a constant fluid pressure at each actuator piston and thereby reduce the tendency of a friction element to premature become "stuck" on a brake drum or rotor high spot.
U.S. Pat. No. 5,161,864 discusses various types of prior art accumulators which have been added to automobile brake systems in order to reduce pressure surges as well as the disadvantages of such systems. The invention described therein provides an adjustable pressure control device for a vehicle hydraulic brake system with a housing having a brake fluid port and a cavity for receiving a diaphragm. A resilient diaphragm is mounted within the cavity, and a fluid passage in the housing connects the fluid port to the cavity to enable brake fluid to pass to one side of the diaphragm. A cap having a control chamber formed therein is affixed to the body such that it overlies the opposite side of the diaphragm. A removable variably-shaped volume control element is mounted within the control chamber in contact with the opposite side of the diaphragm. The non-resilient control element cooperates with the resilient diaphragm to control the volume of the diaphragm which enters into the control chamber and the rate at which the diaphragm collapses into the control chamber around the control element.
The '864 Patent acknowledges that the pressure control device disclosed therein can accommodate a wide range of vehicles having different fluid line pressures only by modification of particular structural elements of the device such as orifice sizes, the diaphragm, or the control element. While the device enables the pressure control device to be custom designed for a narrow range of anticipated pressures, it requires structural modification of one or more elements of the device in order to enable it to handle a different narrow range of anticipated pressures. Thus, the prior art fails to disclose a device which can handle a wide range of anticipated different pressures without any elements of the device undergoing structural modification.
Furthermore, it has now been found that the device of the '864 Patent did not operate as intended when exposed to the high pressure pulsations arising during extreme or panic braking. During very high pressure pulsations, the resilient diaphragm was found to press against the chamber wall and, thus constrained, the diaphragm could no longer oscillate sufficiently to maintain an anti-lock reaction, as necessary to produce braking efficiency. While diaphragms of higher resiliency tended to resist this phenomenon, they exhibit inferior low pressure pulsation performance because the stiff diaphragm did not permit sufficient oscillation to counter the fluid displacements.
Accordingly, it is an object of the present invention to provide a pressure control device which can handle a wide range of anticipated pressures without being structurally modified and without experiencing any performance loss such as restriction of diaphragm oscillation.
Another object is to provide such a pressure control device for a hydraulic brake system.
A further object is to provide such a pressure control device which nullifies hydraulic brake fluid pulsations over a wide range of pressures.